MICROSTRUCTURE AND RELIABILITY OF LASER JET SOLDER BALL BONDING SOLDER JOINTS

Laser jet solder ball bonding (LJSBB) technology is a special soldering method developed in cooperation by Fraunhofer Ltd and PacTech Ltd in 1998 to fulfill the specific needs in the packaging of micro-electro-mechanical system (MEMS) and other functional micro-devices. However, the metallurgy characteristics and reliability of the solder joint produced by LJSBB has not been broadly reported. In the present paper, the commonly used Au/Ni/Cu soldering pads were bonded by LJSBB with Sn3.0Ag0.5Cu lead free solder. The effect of laser energy on the interfacial microstructure of solder joint was investigated by SEM and EDS. The reliability of solder joints was also evaluated by thermal cycle treatment and strength test. Results show that due to the limited heat provided by laser beam, the Au layer of the commonly used Au/Ni/Cu soldering pad cannot completely dissolve into solder during LJSBB process, which leads to the formation of multilayered reactive structure of Au+AuSn+AuSn2+AuSn4 in the interface of solder joint. The AuSn4 locates at the most outside of the reactive structure and presents a needle-like shape. In despite of the brittle nature of Au-Sn intermetallics, the as-soldered joints present a comparatively high strength. However, the strength of solder joint decreases significantly after 100 thermal cycle treatment and keeps nearly unchanged after even more thermal cycle treatment. Metallurgy analysis of joint interface reveals that the thermal cycle treatment plays the roles of promoting the dissipation of residual Au in joint interface through solid state inter-diffusion, and leading to the formation of a quaternary phase of (Au, Ni, Cu, Sn) in interface, which results in weakening of LJSBB solder joint.